Chromium (VI) adsorption onto activated kraft lignin produced from alfa grass (Stipa tenacissima)

Abstract

Activated lignin having a surface area of 1023 m 2 g -1 has been prepared from sulfate lignin that was treated by 30% H 2O 2 and carbonized at 300 oC in order to test the chromium (VI) adsorption from aqueous solution. The influence of contact time, pH, initial concentrations of adsorbent and adsorbate, and temperature on the adsorption capacity were investigated. The maximum removal of Cr(VI) was found to be 92.36 % at pH=2 and a contact time of 80 min. Optimal concentration of lignin and Cr(VI) were found to be 3.8 g L -1 and 180 mg L -1, respectively. The adsorption kinetics data fitted well with a pseudo-second-order equation, and the rate of removal of chromium was found to speed up with increasing temperature. Activation energy for the adsorption process was found to be 18.19 kJ mol -1. The Langmuir and Freundlich adsorption isotherm models were applied to describe the isotherm and isotherm constants for the adsorption of Cr (VI) on lignin. These constants and correlation coefficients of the isotherm models were calculated and compared. Results indicated that Cr (VI) uptake could be described by the Langmuir adsorption model. The maximum adsorption capacity (q m) of Cr (VI) on lignin was 75.75 mg g -1 at 40°C. The dimensionless equilibrium parameter (R L) signified a favorable adsorption of Cr (VI) on lignin and was found to be between 0.0601 and 0.818 (0<R L<1). The thermodynamic parameters such as ΔG°, ΔS°, and ΔH° were calculated, and it was found that the reaction was spontaneous and endothermic in nature. This study indicates that lignin has the potential to become an effective and economical adsorbent for removal Cr (VI) from waste water